This invention relates to a clocked direct voltage converter in which the voltage at an output is controlled and an uncontrolled voltage is produced by coupling to an inductive storage element of the converter.
A direct voltage converter having these features is described, for example, in Published European Patent Specification No. 0 048 934. The direct voltage converter described therein concerns down converters with potential separation. The transformer which also serves as the inductive storage element, comprises a secondary circuit in which the voltage for the load is not controlled, but is "pulled along" by another controlled secondary circuit. In general, such uncontrolled connections, which may be provided with a simple succeeding series regulator, are used for feeding active elements of the control device of the converter.
In clocked direct voltage converters provided with a control device, with a small load the so-called "idling" operation can occur, which is characterized in that the currents through inductive storage elements of the converter temporarily become equal to zero. In the "idling" operation, only a small quantity of energy is transmitted from the primary side to the secondary side of the converter. This means inter alia that the part of the time of a period of the switching clock for which the switching transistor of the converter is conducting is made extremely small, or even equal to zero, by the control device of the converter. However, since the switching transistors because of their parasitic capacitances either cannot beome conductive at all or can become conductive for a minimum amount of time (this minimum amount of time lies for bipolar transistors at 200 ns and for MOS FET's at 100 ns), the following disadvantages ensue therefrom for the "idling" operation:
If the control device does not render the switching transistor conductive at all for one or several clock periods, the switching processes obtain another frequency spectrum; considerably lower frequencies than in normal operation occur in this case. Therefore, the low-pass filters at the converter input, which should protect the supply mains, to which the converter is connected, from interferences, no longer fulfil their function. Moreover, if, for example, the switching frequency lies at 20 kHz in normal operation, the switching process can become audible. When the switching transistor is opened, the voltage at the controlled output exceeds - because of the excessively large minimum amount of time - its nominal value to a considerably greater extent than in normal operation. Therefore, the voltage fluctuations are considerably larger in the "idling" operation than in normal operation.
Because of these disadvantages, operation of the converter in the "idling" mode should be avoided as far as possible. According to DE PS No. 2359555, the "idling" operation is avoided in the case of down converters in that a base load, i.e. a resistor, is connected parallel to the so-called flywheel diode of the down converter by means of a switch, which is operated in pushpull arrangement with the switching transistors.